The invention generally relates to a turbo machine. More specifically, the invention relates to the configuration of the surface structure of heat shields built into a turbo machine.
In general, components involved in relative motion in turbo machines cannot be sealed using contact seals. There is thus a leakage gap between a heat shield and a blade tip, for example. The leakage via this gap has negative effects on the power and efficiency of the turbo machine. However, reducing the size of the leakage gap increases the risk that the blade and the heat shield will scrape against one another, leading to machine failure with serious consequences. Numerous measures were therefore taken in the past to construct these components in such a way that scraping could be tolerated. A hard/soft friction pair is produced by means of special contact coatings and contact tips, one of the friction partners abrading or plastically deforming the other if scraping occurs. In this way, scraping is as if it were buffered, but the leakage gap is permanently enlarged.
The use of conventional elastic means, such as brush seals, fails as soon as they are used in the first stages of a gas turbine and indeed also in the high-pressure part of a modern turbo compressor: in this case, it is only possible to use components which, on the one hand, are resistant to high temperatures and, on the other hand, allow efficient cooling of all components exposed to hot gas.
It is the aim of the present invention, in the case of a turbo machine of the type stated at the outset, to specify means by which leakage gaps between components involved in relative motion can be reduced, which means must furthermore meet the conditions that they accommodate scraping of the components within reasonably to be expected limits by means of purely elastic deformation, such that, after scraping, the leakage gap is not permanently enlarged due to plastic deformation. At the same time, it must be possible to produce these means from materials which are resistant to high temperatures, and efficient cooling of the means must be possible if necessary.
This is achieved by means of the features of the present invention.
The essence of the invention is therefore to provide the surface of a heat shield situated opposite a component involved in relative motion with a microstructure, such microstructures being known per se from coating technology and helping there to produce a more intimate connection between a substrate and a layer. In the present application, however, the microstructure is used directly as a surface of the component. In this context, the microstructure is designed in such a way that its elements have low dimensional rigidity in the direction of the relative motion. If scraping occurs, the elements of the microstructure can then yield, this yielding movement being accommodated purely by elastic deformation. It is thus possible to design leakage gaps to be smaller, and any scraping which may occur does not lead to permanent enlargement of the leakage gap. It is advantageous, especially when using the invention in gas turbines, to provide the heat shieldxe2x80x94or some other component which is provided with a microstructurexe2x80x94with means which allow a cooling medium to be fed to the microstructure.
In a leakage gap, a special embodiment of the microstructure is preferred. In this case, the microstructure element takes the form of a plateau which is offset from the component underneath, that is to say the heat shield for example, and which is connected to the component by a rib. The rib, for its part, is a thin plate which is set on edge on the component and is preferably aligned with its surface normal to the expected leakage flow. The plateau and the rib then have the shape of a xe2x80x9cTxe2x80x9d in section. When used on a heat shield, the xe2x80x9cTxe2x80x9d-shaped configuration can be seen when looking in the axial direction of the turbo machine according to the invention. Overall, this configuration has the following advantages: with their large surface, the ribs obstruct a leakage flow directed from the blade delivery side to the blade intake side, that is to say essentially in the circumferential direction, obstructing it to a large extent though not completely. The obstruction of leakage can be further improved by a combination of different structural elements; the optimum structure for this purpose must be determined as appropriate for the particular case. By virtue of the use of the plate-shaped ribs in their specific orientation, the microstructure elements do have a high-strength in the axial direction of the turbo machine but, in the circumferential direction, a rib includes essentially only of the neutral bending axis. This results in a low dimensional rigidity, in particular a low moment of resistance of the microstructure elements to bending in the circumferential direction of the machine, and a very large bending range in which only elastic but no plastic deformation occurs. It may furthermore prove to be advantageous not to arrange the ribs in a precisely perpendicular manner on the component but to tilt them slightly in the expected direction of scraping, ensuring easier deformation. Microstructure elements of this kind can not only buffer any scraping in a particularly suitable manner, like the contact coatings described in the introduction, but scraping of the blades against the heat shield is literally cushioned. By virtue of the plateaus, in turn, the microstructure forms cavities between its surface and the actual surface of the heat shield, these cavities being well delimited from the leakage flow precisely by the plateaus. On the one hand, this prevents hot gas ingress into the microstructure and, on the other hand, coolant which is introduced into the cavities is used very efficiently.
A slight variation in the xe2x80x9cTxe2x80x9d-shaped microstructure elements is obtained if the plateau is arranged in an edge region of the rib. These microstructure elements take the shape of a capital Greek letter xe2x80x9cgammaxe2x80x9d (xcex93). These microstructure elements essentially have the same advantages as the xe2x80x9cTxe2x80x9d-shaped microstructure elements. A particularly high degree of security against jamming of the microstructure elements in the event of scraping is obtained especially if the ribs slope toward the side on which the plateaus project.
The microstructure described on a heat shield thus presents only very little resistance to scraping of a blade tip, thus avoiding damage to the blade in the event of scraping. The microstructure, in turn, is deformed only elastically. When the cause of scrapingxe2x80x94generally an unfortunate combination of differential thermal expansionsxe2x80x94is no longer present, the microstructure reassumes its original shape, and the leakage gap has not been permanently enlarged. A special shape of the microstructure elements obstructs any leakage flows that may arise within the microstructure, reducing them to a minimum. Good coolability of the structure ensures a long service life, even in the case of extreme ambient conditions.
Microstructure elements with a dovetail cross section have essentially the same properties as the structures with a xe2x80x9cTxe2x80x9d-shaped cross section described above, and can in a certain sense be regarded as a variant of this embodiment.
The extent of the microstructure over the component must of course be chosen by the person skilled in the art in accordance with the application. A xe2x80x9cverticalxe2x80x9d structure dimension in a range of from 1 to 5 mm above the actual surface of the component has proven to be advantageous on a heat shield.
In another preferred embodiment, the microstructure comprises rod-shaped elements, which are arranged in a tetrahedral shape for example. The rod-shaped elements can also serve as a support structure for an essentially two-dimensional grid arranged thereon. A diamond-shaped structure has proved to be advantageous for such a grid. The size of two obtuse angles enclosed in a diamond is advantageously between 90xc2x0 and 160xc2x0. The grid should preferably be arranged in such a way that the expected direction of scraping represents the angle bisector of the obtuse angles. In this arrangement, the diamonds have a low dimensional rigidity and can be practically folded up by a scraping component. Irrespective of the shape of the microstructure, the microstructure elements can be coated. Primary consideration will be given here to ceramic protective layers whose thickness is significantly less than the dimension of the structure, the structured surface thus being retained, these layers serving as a protection against oxidation, for example.